1. Field of the Invention
This invention relates to an automotive vehicle hydraulic system for use with power steering and brake booster units including open-center type hydraulic control valves.
2. Description of the Prior Art
It is an inherent characteristic of an automotive vehicle to be turned with a lighter force on the steering wheel when running at higher speeds. For stable steering operations over the full range of vehicle speeds, it is required for the power steering unit to have a large supply of hydraulic fluid for allowing easy turning of the steering wheel at low vehicle speeds and a small supply of hydraulic fluid for suppressing over-turning of the steering wheel at high vehicle speeds. However, if the brake booster unit has a large supply of hydraulic fluid at low vehicle speeds and a small supply of hydraulic fluid at a high vehicle speeds, the brakes will have less effect at high vehicle speeds. For stable braking operations over the full range of vehicle speeds, it is required for the brake booster unit to have a constant supply of hydraulic fluid.
In conventional automotive vehicle hydraulic systems including power steering and brake booster units, two pump means have been required, one adapted to supply to the power steering unit hydraulic fluid in amounts varying in accordance with vehicle speeds and the other adapted to supply a constant amount of hydraulic fluid to the brake booster unit, resulting in a large-sized and expensive system.